I don't have reproduction steps but I noticed this while someone was using Web Inspector in a WWDC lab. I think the popover also remains on screen on the wrong location if the layer it was attached to moved to a new spot.
Created attachment 212987 [details]
Attached a test case, steps to reproduce:
1. open the inspector
2. show the Styles sidebar
3. select the sole layer in the document
4. wait a bit for a second layer to appear (5s after load)
At this point, notice how your existing layer has shifted down but the popover hasn't.
Created attachment 212988 [details]
Comment on attachment 212988 [details]
Clearing flags on attachment: 212988
Committed r156673: <http://trac.webkit.org/changeset/156673>
All reviewed patches have been landed. Closing bug.